If it was an early ultrasound, then your EDD (estimated due date) was moved to a more accurate date. Especially if your Sept. 5th due date was going off your last menstrual cycle. Ultrasounds are much more reliable and accurate than dating alone.
But remember, an EDD is just that - it's estimated. Your baby can arrive anytime 2 weeks before or after that date and it's perfectly normal. In fact, some babies that are forced out on their "due date" actually don't fair as well as they could have if they'd had that extra bit of time to cook.
If your little one doesn't pop out on your EDD, try not to stress too much. I understand how uncomfortable you must be (trust me, I remember those days!) but try to keep your tiny one in mind and let them cook for as long as possible.

Good luck!